In a number of applications, it is necessary to guide a probe to a target through a solid medium. An example of such an application is in the field of coal bed methane gas (CBM) extraction. While the invention has been specifically developed for this application, it could be used in other applications with few, if any, modifications. The invention is therefore not limited to such an application and those skilled in the art will readily appreciate the applicability of the invention to other fields of use.
In one CBM extraction method, a vertical well is drilled from the surface down through the target coal seam. A pump maintains low pressure in a sump cavity below the seam at the bottom of the well. A lateral hole is drilled horizontally through the coal seam with the intention of intersecting the well. The pump is then used to extract methane from the coal seam. The lateral hole enters the ground from a surface location 300 to 1500 meters in horizontal distance up dip from the vent well. Once in the coal seam the drill string is turned to a more horizontal attitude but following the dip of the coal seam. Due primarily to cumulative systematic errors introduced by the measurement systems, an ellipse of uncertainty is created. In effect, there is a very small chance of the lateral hole intersecting the borehole on a first pass of the drill string.
As a result, it a very hit and miss affair to cause the lateral hole to intersect the borehole and, to date, repeated passes of the drill string have been required to achieve this objective. It will be appreciated that it is very costly to operate a drill rig and each pass of the drill string is therefore very costly not to say time-consuming. Each time a further pass of the drill string is required, the drill string needs to be retracted and a new trajectory plotted and drilled.